Add and verify the text-to-image path, and fix the UniPC flow_shift selection
it exposed.
Bug: the stage chose flow_shift by task (`3.0 if is_t2i else 10.0`). The
framework selects it purely by the named resolution bucket
(`OmniSampleArgs._RESOLUTION_SHIFT_DEFAULTS`, 8B backbone: 256->3.0, 480->5.0,
720/768->10.0); T2V/I2V/T2I share a shift at a given resolution. The task-based
rule only coincidentally matched (T2V@720, T2I@256). The canonical Cosmos3 T2I
is 960x960 (the "720" bucket -> 10.0), so `is_t2i->3.0` was wrong for real T2I.
- Replace with `_flow_shift_for_resolution(h, w)` mapping the longest side to the
framework bucket (<=320:3.0, 640-832:5.0, 960-1360:10.0), applied to all tasks.
- Parity: `test_cosmos3_flow_shift_parity.py` checks the mapping against the
framework's `{VIDEO,IMAGE}_RES_SIZE_INFO` x `_RESOLUTION_SHIFT_DEFAULTS` for
every (resolution, aspect) in the 8B rows (20 cases).
- Update the call-graph mode-dispatch tests for resolution-based shift.
Also harden `_image_to_video_tensor`'s tensor branch to respect the FastVideo
[-1,1] convention for already-preprocessed conditioning tensors (the PIL path,
used by the real pipeline, stays framework-exact and bit-parity-tested).
Example: `examples/inference/basic/basic_cosmos3_t2i_new_api.py` (num_frames=1,
960x960). Verified on B200 (real weights, 35 steps): a coherent red-panda image
matching the prompt, flow_shift=10.0. Full cosmos3 suite: 118 passed, 0 skipped.
Wire up and verify the Cosmos3 image-to-video path. The denoise stage already
handled the I2V conditioning math (encode -> condition mask -> keep frame 0
clean -> velocity zeroed on condition frames, all matching the framework), but
the conditioning pixel clip was built wrong.
Bug: `_image_to_video_tensor` zero-filled every frame after frame 0. The
framework (`cosmos_framework.inference.vision.build_conditioned_video_batch`)
instead fills frame 0 with the image and **repeats the last conditioning frame**
for the rest of the clip (a static video) before VAE-encoding. Because the Wan
VAE is temporal (4x), latent frame 0 (the kept-clean condition frame) depends on
several pixel frames, so zero-filling produces a wrong conditioning latent.
Fix `_image_to_video_tensor` to be framework-faithful:
- aspect-preserving resize + center crop + uint8 quantization, then `/127.5 - 1`
(mirrors `load_conditioning_image` / `_resize_and_center_crop`);
- static-repeat fill across all frames (mirrors `build_conditioned_video_batch`).
Parity: add `test_cosmos3_i2v_conditioning_parity.py` comparing the FastVideo
conditioning clip against the framework's `load_conditioning_image` +
repeat-fill across aspect/size/frame-count cases — bit-exact (max abs diff 0.0).
Example: `examples/inference/basic/basic_cosmos3_i2v_new_api.py` (new
VideoGenerator API, image via `InputConfig(image_path=...)`,
`assets/images/cyclist.jpg` default).
Verified on B200 (1280x704, 29f, 35 steps, real weights): frame 0 reproduces
the conditioning image and the clip generates coherent forward motion following
the prompt. Full cosmos3 suite: 98 passed, 0 skipped.
The first real-weights Cosmos3-Nano T2V run produced an all-black video.
Root cause: the checkpoint's diffusers-style scheduler_config.json sets
use_karras_sigmas=true, and FastVideo's vendored UniPC checks karras
*before* use_flow_sigmas, so it built diffusion (beta) sigmas instead of
flow-matching sigmas -> scheduler.step diverged to NaN latents -> black
frames. The DiT/CFG velocity itself was clean.
The framework samples with FlowUniPCMultistepScheduler (pure flow:
shift + num_train_timesteps only). FastVideo's vendored UniPC is the same
algorithm, so the fix is config, not a port:
- Coerce the loaded scheduler to the flow setup in
Cosmos3OmniDiffusersPipeline.initialize_pipeline (kill karras/exp/beta
sigmas, force use_flow_sigmas + flow_prediction + final_sigmas_type=zero)
and rebuild the runtime scheduler from it.
- Use FastVideo's native UniPC (not diffusers) in the pipeline and tests,
honoring the "no diffusers at runtime" constraint.
Parity:
- Add test_cosmos3_scheduler_parity.py: FastVideo vendored UniPC (flow
config) vs the framework FlowUniPCMultistepScheduler. timesteps bit-exact,
sigmas within fp32 epsilon (~1e-8), full multi-step trajectory < ~1e-6
across shift in {10 (video), 3 (t2i)} and steps in {4,10,35}.
- Fix test_cosmos3_denoise_cfg_parity to use the framework scheduler as the
oracle (it previously compared diffusers-vs-diffusers, so the scheduler
was never actually checked against the framework).
Integration fixes to run end-to-end on real weights:
- registry: map checkpoint DiT class Cosmos3OmniTransformer ->
Cosmos3VFMTransformer.
- component_loader: route the "text_tokenizer" module to TokenizerLoader;
filter scheduler config to the class's __init__ params (robust to
diffusers schema drift like shift_terminal/sigma_min/sigma_max).
- DiT: materialize_non_persistent_buffers (recompute rotary_emb.inv_freq
after meta-device FSDP load) + cast vision latents / timestep embeds to
the compute dtype (no-ops in the fp32 parity tests).
- sequence_packing: move packed tensors to the model device in to_dit_kwargs.
- config: drop the unused identity text-preprocess (Cosmos3 has no text
encoder; the DiT tokenizes in the denoise stage).
Adds examples/inference/basic/basic_cosmos3_new_api.py (new VideoGenerator
API). Verified: 1280x704, 29 frames, 35 steps on B200 -> coherent video
matching the prompt (no NaNs, real pixel variance). Full cosmos3 parity
suite green.
Adds 8 CPU-only parity test files + conftest stubs (StubScheduler, StubCosmos3VAE, StubCosmos3Transformer) under tests/local_tests/cosmos3/ mirroring the vllm-omni reference at tests/diffusion/models/cosmos3/conftest.py. All 15 tests skip until the FastVideo modules land (subsequent commits). Reference: vllm-omni PR #3454 @ 8536f5b1421f.
SolitaryThinker